1. Field of the Invention
This invention relates to a pulse generating apparatus, and more particularly, to a pulse generating apparatus for generating pulses used to descramble a scrambled video signal in a cable television system for example.
2. Description of the Related Art
Cable television (CATV) is a toll broadcasting service and only subscribers can view its programs. The CATV video signal has its sync signal portion scrambled so as to preclude non-subscribers from viewing the programs, and subscribers' CATV receiver sets are equipped with a device for descrambling the scrambled sync signal.
A conventional descrambling circuit will be described with reference to FIG. 7. The block diagram in FIG. 7 shows only the pulse generating apparatus which produces pulses for descrambling the scrambled sync signal, and the input signal 1 is a AM audio signal in the television signal. In this conventional system, the audio signal is amplitude modulated by a signal used for descrambling the video signal, and the descrambling signal is detected from the amplitude modulated (AM) audio signal. The descrambling signal is added to the video signal so that the scrambled sync signal is descrambled, and then the video signal is made normally visible by an ordinary TV set. In FIG. 7, symbol 1 denotes an AM signal input, 2 is an AM detector, 3 is an AM detector output, 4 is an integrator, 5 is a reference voltage, 6 is a threshold voltage, 7 is a comparator and rectangular pulse generator which compares the AM detector output with the threshold voltage, and 8 is a rectangular pulse output which is delivered to the video signal processing portion of the CATV receiver to descramble the scrambled video signal.
The AM signal input 1 is detected by the AM detector 2, and the resulting AM detector output 3 is fed to one input of the pulse generator 7. The AM detector output 3 is separately integrated by the integrator 4, which output is added by the reference voltage 5 to form the threshold voltage 6, and it is fed to another input of the pulse generator 7. The pulse generator 7 produces a rectangular pulse when the voltage of the AM detector output 3 is higher than the threshold voltage 6. The integrator 4 provides an average voltage of the AM detector output 3 to shift the threshold voltage 6, thereby compensating the fluctuation of the rise timing of rectangular pulses caused by a change in the level of the AM signal input 1 due to transmission conditions or the like.
In the foregoing conventional circuit arrangement, the fluctuation of the AM signal peak level is followed by the d.c. level of the integrator output. Therefore, the displacement of the pulse rise timing of the rectangular pulse can be corrected. However, in case the AM modulation depth differs among transmitters due to their accuracy or the like, only the peak level of the AM detector output varies differently on each transmission channel, with the d.c. output voltage level of the integrator 4 being unchanged, and therefore the displacement of the pulse rise timing cannot be corrected. A significant timing displacement causes the video signal portion to receive the rectangular pulse output at an erroneous timing, and such inaccurate descrambling operation cannot reproduce well-synchronized pictures. The matter will further be described in connection with FIG. 8.
FIG. 8(a) is the case of the standard AM modulation depth. For the standard AM modulation depth to have a peak-to-bottom voltage ratio (between the portions 1A and 1B) of 6 dB for example, the reference voltage is determined so that rectangular pulses are produced at a 50% AM detector output. In the figure, symbol 1C denotes the threshold voltage, 1D is the AM detector output, and 1E is the rectangular pulse.
FIG. 8(b) is the case of a greater modulation depth than standard, showing that the threshold voltage does not follow the varying AM modulation depth, and the rectangular pulses are generated at the AM detector output below 50%, resulting in a leading pulse rise timing as compared with the case of FIG. 8(a).
FIG. 8(c) is the case of a smaller modulation depth than standard, showing the generation of the rectangular pulses at the AM detector output above 50%, resulting in a lagging pulse rise timing as compared with the case of FIG. 8(a).
The cases of FIGS. 8(b) and 8(c) fail to achieve the accurate descrambling and do not reproduce correct, synchronized pictures.